In multiple-input-multiple-output (MIMO) systems, there is an antenna-to-antenna coupling which causes an antenna voltage standing wave ratio (VSWR) to degrade significantly (e.g. 2.5:1, worst case). Use of techniques such as beam steering in such MIMO systems can also degrade the antenna VSWR. Still yet, bandpass filters [e.g. film bulk acoustic resonator (FBAR) filters, etc.] are typically placed before each antenna, and these filters have VSWRs in the order of 2:1 (worst case). Thus, the cascaded VSWR of the aforementioned filter and antenna can be as bad as 5:1 in some cases for all angles (i.e. all antenna beaming angles between zero (0) and three-hundred and sixty (360) degrees, etc.).
In the past, isolators have been inserted between a power amplifier and the aforementioned filter/antenna, in order to protect the amplifier from failing due to a detrimental VSWR. Unfortunately, isolators are expensive and take up a lot of area. In addition, isolators are generally not amenable to integration (on an integrated circuit). Event still, when employing isolators, approximately half of a transmit power may be lost due to a mismatch between an isolator output and the aforementioned 5:1 cascaded VSWR at the filter input.